Aromatic hydrocarbons, such as benzene, toluene, and xylene can be recovered from various hydrocarbon feed stocks including catalytic reformates, light by-products from various hydrocarbon process units such as xylene isomerization, by-product oil from the manufacture of coke, and hydrogenated pyrolysis gasolines that also contain non-aromatic hydrocarbons. Use of an aromatic selective solvent can facilitate the removal of the aromatic from non-aromatic hydrocarbons in, e.g., an extractive process. In some processes, the hydrocarbon feed stock can be contacted with an aromatic selective solvent, in an extractive distillation column (ED column), which may include the presence of water. The ED column produces a raffinate phase, including one or more non-aromatic hydrocarbons and water, if present, which can exit the overhead of the column. A rich solvent phase including the aromatic selective solvent, and one or more aromatics in the feed stock can exit the bottom of the ED column. The rich solvent phase is separated in a second distillation column, which may include the presence of water, to produce an extract overhead stream of the desired aromatic hydrocarbon components and a lean solvent stream that is depleted in aromatic hydrocarbons and is recycled to the ED column.
To meet purity and recovery requirements for the desired aromatic hydrocarbon products, it is important to obtain a good separation between the lightest aromatic and the heaviest non-aromatic hydrocarbons in the ED column. Ideally, the aromatic selective solvent retains all of the lightest feed aromatic, e.g. benzene, in the lower portion of the ED column while all of the heaviest non-aromatics are driven out of the upper portion of the ED column. Therefore, it can be more difficult to meet stringent purity and recovery requirements when the feed stream has a wider boiling range of mixed hydrocarbon species. It has also been recognized that various steps may be taken to remove contaminants in the rich solvent stream produced by the ED column. Otherwise, these contaminants, such as aromatics which are heavier than the desired aromatic products and non-aromatics which are not driven into the raffinate stream, can accumulate in the lean solvent stream produced in the second distillation column as the solvent is recycled in the process. Contaminants may be introduced to the solvent via mechanical leaks in the equipment which could introduce items such as heating mediums or lubricating oils. Contaminants can also be formed within the process such as by degradation of the aromatic selective solvent.
It is known to contact a portion of the lean solvent produced by the second column with a secondary aqueous solvent, e.g. water, with and without a portion of the non-aromatic raffinate stream, to produce a secondary aqueous solvent stream containing the aromatic selective solvent, which is further separated before being recycled to the ED column. See, for example, U.S. Pat. No. 3,642,614 and US 2009/0038991 A1. There remains a need in the art for improved apparatus and processes that enable the removal of contaminants from the lean aromatic selective solvent stream used in extractive distillation processes.